This application is based on and hereby claims priority to German Application No. 100 47 583.3 filed on Sep. 26, 2000, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The invention relates to a magnetic bearing.
2. Description of the Related Art
Magnetic bearings allow moving parts to be mounted free from any contact or wear. They therefore do not require any lubricants and can be of a low-friction design. What are known as active magnetic bearings, with electromagnets, require an active position control, which controls the currents of the supporting magnets and counteracts deviations of the rotor body from its intended position by position sensors and a control loop. What are known as passive magnetic bearings, on the other hand, stabilize their position by themselves, so that no active position control is necessary.
U.S. Pat. No. 4,072,370 A discloses a passive magnetic bearing with an arrangement of permanent magnets both on the stator and on the rotor.
Furthermore, there are known passive magnetic bearings in which only one of the bearing parts is formed by permanent-magnetic elements and the other bearing part includes a superconductor. The permanent-magnetic elements induce shielding currents when there is a change in position, as a consequence of changes in the field in the superconductor. The resulting forces may be repelling or attracting, but are always directed in such a way that they counteract the deflection from the intended position. Consequently, an inherently stable bearing can be achieved and it is possible to dispense with a complex and fault-susceptible control. However, cooling of the superconductor material is required. Magnetic bearings with superconductors are described, for example, in U.S. Pat. No. 5,196,748 A and EP 0 322 693 A.
DE 44 36 831 C2 then discloses a further passive magnetic bearing with a high-temperature superconductor. This known magnetic bearing includes a first bearing part, which is connected to a rotor shaft, and a second bearing part, which is arranged on a stator and surrounds the first bearing part. One of the two bearing parts has the high-temperature superconductor. The other bearing part includes an arrangement of permanent-magnetic elements arranged adjacent to one another. The magnetization of neighboring permanent-magnetic elements is opposite to one another. The interspaces between respective pairs of permanent-magnetic elements are filled with ferromagnetic material to concentrate the magnetic flux emerging from the permanent-magnetic elements on the side facing the other bearing part. As a result, great bearing rigidity (stability) is obtained.
In a configuration according to DE 44 36 831 C1, the permanent magnets are provided in a hollow-cylindrical arrangement on the inner bearing part and the superconductor is arranged as a hollow-cylindrical structure on the inner side of a hollow-cylindrical support body of the outer bearing part. Cooling channels for passing through liquid nitrogen are formed in the support body to cool the superconductor. In another configuration, the high-temperature superconductor is arranged on the inner bearing part on the rotor shaft, a cooling channel being provided in the rotor shaft for the liquid nitrogen to cool the high-temperature superconductor. The permanent-magnetic elements with the ferromagnetic intermediate elements may be axially arranged in relation to the rotor shaft one behind the other in the form of thin rings or else be axially stretched out and arranged one behind the other in the circumferential direction, respectively with alternating magnetization.
In DE 44 36 831 C2, a material with an energy product (B*H)max of at least 20 MGOe, in particular a neodymium(Nd)-iron(Fe)-boron(B) alloy or a samarium (Sm)-cobalt (Co) alloy, is proposed as the permanent-magnetic material. The permanent-magnetic material may also be cooled to increase its coercive field strength.
The permanent-magnetic materials used in the known magnetic bearings to achieve the high magnetic fields are themselves exposed to considerable forces by the high magnetic fields. With the permanent magnets arranged on the rotating bearing part, centrifugal forces additionally act on the permanent magnets. These forces may then lead to the detachment of individual magnetic particles, in particular from the brittle permanent magnets produced power-metallurgically by sintering or pressing, or even to the rupture of the permanent magnets, in particular in the case of permanent loading and material fatigue. This can, however, bring about considerable damage or even complete destruction of the magnetic bearing.
EP 0 728 956 A1 then discloses a magnetic bearing with a superconductor on the stator and an arrangement of permanent magnets on the rotor and also a bearing gap between the superconductor and the permanent magnets. The permanent magnets are formed in an annular manner and arranged concentrically in relation to the axis of rotation of the rotor. Annular, soft-magnetic flux guiding elements are provided between the individual annular permanent magnets. The annular permanent magnets, on the other hand, are formed from a magnetic sintered material with a high energy product, in particular a samarium-cobalt or a neodymium-iron-boron magnetic material. In order to prevent a rupture of these sintered permanent magnets, in particular at high rotational speeds, a reinforcing ring of a glass-fiber or synthetic-fiber reinforced plastic is then arranged around the outermost ring of the arrangement of magnets and radially holds the arrangement of the annular permanent magnets together under a radial pressure.
A similar arrangement is also known from Patent Abstracts of Japan with respect to JP 09 049 523 A.
EP 0 413 851 A1 discloses a bearing ring for magnetic bearings for use in magnet-mounted vacuum pumps with arrangements of permanent magnets on the rotor and the stator. Iron-neodymium-boron magnets or cobalt-samarium magnets are proposed as permanent magnets. This known magnetic bearing includes, on the rotating shaft, bearing rings which respectively include a hub ring, a ring of permanent magnets and a reinforcing ring. The reinforcing rings have the task of avoiding destruction of the permanent rings being caused by the high centrifugal forces and preferably are formed of high-grade steel.
Patent Abstracts of Japan with respect to JP 08 200 368 A further discloses a magnetic bearing with a superconductor on the outer-lying stator and an arrangement of permanent magnets on the inner-lying rotor. The arrangement of permanent magnets includes a number of permanent magnets in the form of segments of a ring lying adjacent to one another in the circumferential direction and combining with one another to form a closed ring. Arranged around the outer periphery of all the ring-segment-like permanent magnets is a holding ring, to avoid the permanent magnets being ruptured by the great centrifugal forces at high rotational speeds. Also provided is a thrust ring, which presses the holding ring radially inward against the outer faces of the permanent magnets. The permanent magnets are not spaced apart from one another but touch one another. Flux guiding elements are not provided between the permanent magnets.
In the case of known magnetic bearings in which radial holding devices are provided for the permanent magnets, however, the supporting forces of the bearing or the load-bearing capacity of the bearing are reduced. This is because, in the case of the known magnetic bearings, the radial holding devices are arranged between the permanent magnets and the associated flux guiding elements, if any, on the one hand and the bearing gap on the other hand and, as a result, reduce the magnetic flux density effective in the bearing gap and with the superconductor, since the bearing gap must not become less than a specific minimum width.
The invention is therefore based on an object of protecting the magnetic bearing from destruction or damage of the permanent magnets, without significantly lessening the supporting force of the bearing.
The magnetic bearing preferably includes at least one inner bearing part and at least one outer bearing part, which surrounds the inner bearing part. Arranged on one of the two bearing parts are at least one permanent magnet, in particular of hard-magnetic material, and at least one flux guiding element to guide the magnetic flux of the permanent magnet. The flux guiding element is arranged axially (or: parallel) in relation to the axis of rotation adjacent (or: offset in relation) to the permanent magnet, and generally is formed of magnetically conducting, in particular soft-magnetic and/or ferromagnetic, material. Arranged on the other of the two bearing parts, on the other hand, is at least one structure (coupling or interacting means) which interacts (magnetically) with the permanent magnet or magnets in such a way that a bearing gap (bearing distance, bearing interspace) which runs around the axis of rotation can be or is formed between the inner bearing part and the outer bearing part, so that contact-free rotation of the two bearing parts with respect to each other about an axis of rotation is possible.
A radial holding device (protective device, shielding device) holds and stabilizes the permanent magnet in at least one radial direction perpendicularly in relation to the axis of rotation, but at least toward the bearing gap, and as a result stops the permanent magnet or fragments of the permanent magnet from moving in this radial direction toward the bearing gap. Consequently, the permanent magnet is shielded from the bearing gap by the radial holding device and, as a result, can no longer shed any fragments or particles in this radial direction. Consequently, the permanent magnet can no longer cause any damage to the magnetic bearing even under the influence of strong forces, in particular as a result of high magnetic fields or high centrifugal forces, and a disintegration or detachment of particles possible as a result.
In order not to reduce the supporting force of the magnetic bearing, as further measures, on the one hand each permanent magnet is expanded less far in the radial direction, at least toward the bearing gap, than the flux guiding element or elements and on the other hand the radial holding device is formed with in each case a single holding element for each permanent magnet.
According to the invention, consequently, the radial holding device is arranged between the bearing gap and the permanent magnet and consequently the particularly trouble-prone region of the bearing gap is protected from fragments and detached parts of the permanent magnet by the radial holding device. Each flux guiding element serves for guiding the magnetic flux of the permanent magnets and generally also for concentrating and intensifying it in the bearing gap. The supporting force of the bearing is only slightly affected by the holding device according to the invention. This is because, on the one hand, the permanent magnets are set back, and consequently the holding elements do not protrude, or only little, into the bearing gap, so that the bearing gap therefore does not have to be chosen to be any larger. On the other hand, the flux guiding elements also guide the magnetic flux of the set-back permanent magnets still with virtually the same intensity toward the bearing gap. The supporting force of the bearing is then produced predominantly in the region of the high magnetic flux density and magnetic flux density gradient radially outside on the flux guiding elements. The radial holding device provided in the region of the permanent magnet or magnets does not disturb the magnetic flux at these points of emergence on the flux guiding elements. The invention is in this respect also based on the consideration that it is possible to dispense with stabilization of the flux guiding elements, because the flux guiding elements can be configured to be mechanically more stable than the permanent magnets.
The magnetic bearing according to the invention is generally intended for a body (rotor) which is rotatable or rotates about an axis of rotation. For this purpose, the inner bearing part or the outer bearing part is or can be connected to the rotor.
In a first embodiment, the radial holding device is arranged on an outer side of the permanent magnet, facing away from the axis of rotation, and, as a result, prevents parts of the permanent magnet from escaping in the outwardly directed radial direction. This prevention of movement of the permanent magnet and fragments thereof, at least in the outwardly directed radial direction, by the radial holding device is particularly expedient in the case of an embodiment of the magnetic bearing in which at least one permanent magnet is provided on the rotatable or rotating bearing part or the bearing part which is connected or can be connected to the rotor. This is because, as a result of the rotation of the bearing part and of the permanent magnet located on it, centrifugal forces which are all the greater the faster the bearing part rotates act on the permanent magnet in the outwardly directed radial direction. The radial holding device then holds the permanent magnet, at least in this critical direction of the centrifugal forces. As a result, centrifugal forces in the rotating permanent magnet are also intercepted in the outward direction and any detachment of magnetic particles in the outward direction is avoided.
In a second embodiment, the radial holding device is arranged on an inner side of the permanent magnet, facing the axis of rotation, and, as a result, prevents parts of the permanent magnet from escaping in the inwardly directed radial direction. This is of advantage, for example, if the permanent magnet is arranged on the outer bearing part, in order to protect the bearing gap and the inner bearing part from fragments of the permanent magnet. This second embodiment may also be combined with the aforementioned first embodiment.
In an advantageous embodiment, the radial holding device bears with positive and/or nonpositive engagement against the permanent magnet, at least in a subregion, so that in particular interspaces are avoided and the permanent magnet is held mechanically together and thereby stabilized.
The radial holding device may be formed at least partly by a flexible (bendable) material and can then be easily adapted to differently shaped permanent magnets and bearing parts. At least one holding element of the radial holding device is preferably formed in a strip-shaped manner and in this case generally with its broad side toward the permanent magnet.
The radial holding device may, however, also be formed at least partly by a dimensionally stable (rigid) material.
The material of the radial holding device is generally mechanically stable, resistant to tension and tearing and preferably non-magnetic, in order to lessen the supporting force of the bearing as little as possible or not at all. Preferred materials for the radial holding device are fiber materials or fiber composites. Plastics reinforced with carbon fiber or glass fiber or mineral fiber (polymer materials), woven, knitted or laid fiber fabrics or scrims or pressed fiber materials come into consideration in particular. Furthermore, unmagnetic metals or metal alloys, for example unmagnetic steels, are also suitable.
In an advantageous embodiment, the magnetic bearing includes a number of permanent magnets arranged axially in relation to the axis of rotation (or: in the form of a stack) adjacent to one another on one of the bearing parts. The permanent magnets may, however, also be arranged adjacent to one another in an arrangement running around the axis of rotation, in particular in the circumferential direction. Respectively arranged between at least two of the permanent magnets and/or on the outside of the outer permanent magnets in the axial direction, there is then a flux guiding element. The magnetization of the permanent magnets preferably alternates, that is to say the magnetization of two adjacent permanent magnets has opposite polarity.
In a development, a common holding element may extend over the individual holding elements for the permanent magnets and over the flux guiding elements to provide additional reinforcement.
Furthermore, in an advantageous configuration, at least one radial holding element runs around the axis of rotation in one or more layers, that is to say forms a closed hollow-cylindrical, annular, swirl-shaped or loop-shaped form around the axis of rotation.
The radial thickness (expanse) of the radial holding element or elements, measured from the permanent magnet in the radial direction under consideration, is generally at most one third (⅓), in particular at most one quarter (xc2xc) and preferably at most one tenth ({fraction (1/10)}) of the radial thickness of the permanent magnet. Furthermore, the radial expanse of each radial holding element is preferably also chosen to be less than the radial dimension (gap width) of the bearing gap.
The radial expanse of the radial holding element or elements is preferably also chosen such that it does not significantly exceed the difference between the radial expanse of the flux guiding element or elements protruding further outward, on the one hand, and the radial expanse of the permanent magnet or magnets, on the other hand, and is preferably less than or at most equal to this difference. This allows the bearing gap to remain small and it is limited in its radial dimensions only by the flux guiding elements. The holding elements and the flux guiding elements may end flush with one another, in particular in the radial direction, so that there is formed a substantially smooth, preferably cylindrical, common surface, which bounds the bearing gap.
Since each permanent magnet of the magnetic bearing is stabilized by the associated radial holding element, each permanent magnet may be formed of a brittle material, in particular a sintered or pressed molding, or even not be dimensionally stable at all, in particular be formed from magnetic powder. This allows the magnetic materials neodymium(Nd)-iron(Fe)-boron(B) alloy or samarium(Sm)-cobalt(Co) alloy to be used without the risk of mechanical damage to the bearing.
In an advantageous configuration, the permanent magnet or magnets and/or the flux guiding element or elements and/or the holding element or elements and/or the coupling means surround the axis of rotation in a form which is closed (all around), preferably in the form of a ring. The ring cross section may in this case be, in particular, circular, disk-shaped or rectangular, in a way corresponding to a hollow-cylindrical or toroidal ring form. Consequently, the longitudinal section of the ring perpendicular to the axis of rotation may in particular be shaped in the manner of a circular ring.
The radially inner- or outer-lying outer side of at least one permanent magnet and/or of the associated radial holding device is preferably substantially cylindrically shaped, so that a generated surface of a cylinder is obtained on the outer side of this permanent magnet or an arrangement of a number of such permanent magnets or the radial holding device arranged on it. The outer side of at least one permanent magnet and/or of the associated radial holding device may, however, also rise up, in particular in one direction, for example axially and/or conically. Preferably, the radial holding device is adapted to the outer form of the permanent magnets.
A particularly advantageous configuration of the magnetic bearing is distinguished by the fact that a superconducting structure, which preferably includes a high-temperature superconductor, that is to say a superconductor of which the critical temperature lies above 77 K, is provided as the coupling means. However, electromagnets, which inductively generate a magnetic field, or permanent magnets may also be used as the coupling means. In particular in the case of electromagnets, an automatic position control is then also generally provided.
Furthermore, it is advantageous to arrange the coupling means on the side of the bearing part facing the bearing gap, in order to achieve good efficiency of the coupling.